ntb: Constify struct bus_type

'struct bus_type' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  69682	   4593	    152	  74427	  122bb	drivers/ntb/ntb_transport.o
   5847	    448	     32	   6327	   18b7	drivers/ntb/core.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  69858	   4433	    152	  74443	  122cb	drivers/ntb/ntb_transport.o
   6007	    288	     32	   6327	   18b7	drivers/ntb/core.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>